Colin Bradley

A review of machine vision sensors for tool condition monitoring

Abstract Tool condition monitoring has gained considerable importance in the manufacturing industry over the preceding two decades, as it significantly influences the process economy and the machined part quality. Recent advances in the field of image processing technology have led to the development of various in-cycle vision sensors that can provide a direct and indirect estimate of the tool condition. These sensors are characterised by their measurement flexibility, high spatial resolution and good accuracy. This paper provides a review of the basic principle, the instrumentation and the various processing schemes involved in the development of these sensors.

Segmentation of a wrap-around model using an active contour

Abstract Segmentation in the past has been applied to single range maps, which are capable of modelling only the part of an object that is visible from the camera viewpoint. In the field of reverse engineering, there is a great need for the segmentation of complete wrap-around object models. In this paper, a new method is presented for segmenting a wrap-around wireframe model, which is created by interpolating and merging multiple range maps. A semi-automatic edge-based approach is used for the segmentation. Surface differential properties are estimated at each point in the model, and curvature extrema are identified as possible edge points. An energy-minimizing active contour is used interactively to link the edge points. The designer defines small closed contours within the regions to be segmented, and the contour is ‘inflated’ until it is trapped by the potentials at the edge points.

G1 continuity of B-spline surface patches in reverse engineering

When a clay model or industrial part is reverse engineered, B-spline surfaces are typically fitted to surface data points collected by a coordinate measuring machine, or, in more recent times, a laser scanner. The surface fitting procedure results in a patchwork of B-spline surfaces. Some of these patches may be required to blend smoothly with their neighbours to a specified degree of continuity. Past research on surface patch continuity has focused on Bezier surfaces. In the paper, some of those techniques are modified and applied to B-spline surfaces. An optimization method is presented which can achieve an approximate global G1 continuity of a given patchwork of B-spline surfaces. The method is illustrated by a complex physical model, which is digitized by a 3D laser scanner, and reverse engineered to create a smooth cad model.

Cloud data modelling employing a unified, non-redundant triangular mesh

This paper describes an application of error-based triangulation to very large sets of three-dimensional (3D) data. The algorithm is suitable for processing data collected by machine vision systems, co-ordinate measuring machines or laser-based range sensors. The algorithm models the large data sets, termed cloud data, using a unified, non-redundant triangular mesh. This is accomplished from the 3D data points in two steps. Firstly, an initial data thinning is performed, to reduce the copious data set size, employing 3D spatial filtering. Secondly, the triangulation commences utilising a set of heuristic rules, from a user defined seed point. The triangulation algorithm interrogates the local geometric and topological information inherent in the cloud data points. The spatial filtering parameters are extracted from the cloud data set, by a series of local surface patches, and the required spatial error between the final triangulation and the cloud data. Two procedures are subsequently employed to enhance the mesh: (i) the edges of mesh triangles are adjusted to produce a mesh containing approximately equilateral triangles; and (ii) mesh edges are aligned with the boundaries present on the object to minimise smoothing of naturally occurring features. Case studies are presented that illustrate the efficacy of the technique for rapidly constructing a geometric model from 3D digitised data.

Free-form surface reconstruction for machine vision rapid prototyping

Prototyping is an essential step in the manufacture of many objects, both consumer and industrial. A fundamental step in this process is the definition of the 3-D form of the object shape, for example, a designers model created in clay or wood. A 3-D vision system (range sensor) offers the advantage of speed in defining shapes compared to a traditional tactile sensor. The viability of using range sensors is demonstrated by the development of a rapid prototyping system comprised of a laser-based range sensor and software that creates a computer model of the object. One particularly important area of application is the reverse engineering of models comprised of free-form surfaces. This is important in mold and die manufacture for aerodynamic and ergonomic object surfaces. Several methods of modeling free-form surfaces from irregular data are examined and a radial basis function approach is used in developing a self-contained, interactive software package utilizing the laser-generated range data.

Hot-air optical turbulence generator for the testing of adaptive optics systems: principles and characterization

A statistically repeatable, hot-air optical turbulence generator, based on the forced mixing of two air flows with different temperatures, is described. Characterization results show that it is possible to generate any turbulence strength up to CN2 Dh approximately 6 x 10(-10) m1/3, allowing a ratio of beam diameter to Frieds parameter as large as D/r0 approximately 25 for one crossing through the turbulator or D/r0 approximately 38 for two crossings. The outer scale (L0 approximately 133 +/- 60 mm) is found to be compatible with the turbulator mixing chamber size (170 mm), and the inner scale (l0 approximately 7.6 +/- 3.8 mm) is compatible with the values in the literature for the free atmosphere. The temporal power spectrum analysis of the centroid of the focused image shows good agreement with Kolmogorovs theory. Therefore the device can be used with confidence to emulate realistic turbulence in a controlled manner. A calibrated CN2 profile, both in layer altitude and strength, is necessary for the testing of off-axis adaptive optics correction (multiconjugate adaptive optics). Testing was done to calibrate the CN2 profile using the slope detection and ranging technique. The first results, with only one layer, show the validity of the approach and indicate that a multiple-pass scheme is viable with a few modifications of the current setup.

Reverse engineering employing a 3D laser scanner: A case study

An important step in many manufacturing processes is the definition of the three-dimensional form of an objects shape. A common example is the definition of a designers model created in clay or wood. A three-dimensional laser scanner offers the advantage of speed and non-contact sensing compared to traditional touch probe sensing. The three-dimensional data produced have been used in a multi-patch reverse engineering program in order to model objects where many types of surface patches, such as quadric and free-form, are blended together on one object. The surface reconstruction algorithms have been demonstrated to provide accurate and smooth surfaces that are suitable for CAD database integration and CNC machine toolpath generation. The algorithms have been implemented in an interactive 3D surface modelling program that produces surfaces compatible with current CAD/CAM systems. The application of the technology to the reverse engineering of a typical injection moulded part is described.

A multi-sensor approach to automating co-ordinate measuring machine-based reverse engineering

Abstract Co-ordinate measuring machines (CMMs) have been widely used in industry for reverse engineering. In an ideal reverse engineering system, the object would be automatically digitised, the data would be segmented into constituent surface patches, and an accurate solid model would be generated. However, at present, the time-consuming manual digitisation of many objects discourages the implementation of reverse engineering. As a major step toward attaining this goal, a charged coupled device (CCD) camera and a CMM touch probe digitiser are used together in a novel reverse engineering process. Neural network based stereo image processing is used to locate the object in the CMM work space, and to generate the CMM touch probe path. Precise data is then collected using the CMM touch probe, from which a CAD/CAM model can be constructed.

Curved surface machining through circular arc interpolation

Abstract In curved surface machining it is customary to represent and to generate the surface as a sequence of short linearly interpolated moves. Experimental measurements of such stop-start motion shows that full cutting speed is achieved typically for only ten percent of the machining time: the remaining time is spent in accelerating, decelerating, or pausing between instructions. An algorithm is presented which uses a recursive approach to maximize the number of circularly interpolated moves for surface generation. A reduction in machining time of up to 5:1 is demonstrated using this approach.

Advances in the Field of Reverse Engineering

AbstractThis paper presents an overview of advances in the field of reverse engineering over the last 25 years. Initially, several basic definitions and terminology will be reviewed, followed by a discussion of how and why the foundational technology (both hardware and software) has developed. This will be examined from both a driving research and market demand perspective. The evolution of hardware, such as detectors, cameras, optics and sources will be discussed with particular emphasis on landmark developments that transformed the field. Significant advances in the methodologies and algorithms underlying software tools will also reviewed. A look at the latest developments and achievements in reverse engineering techniques shows the incorporation of knowledge based and CAD driven approaches. The paper will conclude with several state-of-the-art reverse engineering case studies from the fields of computer arts, medicine, dentistry, product development, manufacturing and virtual heritage. The authors cons...